Chemical Engineering October 2010 - 38

Cover Story
and solvents although the PTFEcoated
fabric is more resistant.
Below ambient system design
For below ambient applications, water
vapor condensation inside the insulation
is a major concern, hence, the
insulation system must have a very
low water-vapor permeance**. A system
permeance of less than 0.01 perm
is recommended for chilled-water
systems exposed to outdoor conditions.
The permeance rating for the
materials should be obtained using
ASTM E96, Wet Cut Test Method B
for water vapor transmission, including
the effects of joints. For very low
service temperatures or continuous
low-temperature operation, the use of
so-called " zero permeance " jacketing is
recommended. These generally have a
minimum thickness of 0.006 in. and
consist either of a corrosion resistant,
multi-ply laminate jacketing (possibly
with a pressure sensitive adhesive)
or of low permeability epoxy mastic
applied over an open weave scrim
around the insulation. For cryogenic
systems, thicker, more robust vaporretarder
jacketing is recommended.
This type of jacketing made of either a
polymer bitumen or butyl rubber with
a thickness of up to 0.055 in. is commercially
available. Of course, the use
of properly sealed cellular-glass insulation
can by itself achieve a water-vapor-tight
insulation system on below
ambient temperatures since it has a
water vapor permeability of less than
0.005 perm-inch (so, a five-inch thick
cellular glass system would provide a
permeance of less than 0.001 perm).
Regardless of which combination
of jacketing and insulation materials
are selected to provide the low permeance,
the installation details and
system maintenance are critical to the
longterm thermal performance of the
system. Once the system has started
to absorb significant quantities of perhaps
greater than 2 vol.%, then the
system performance will probably be
unsatisfactory due to increased insulation
thermal conductivity. Water vapor
transmission (WVT) for a given time
** Permeance is the rate of water vapor transmission
between two surfaces of a material for
specific conditions. The U.S. unit of permeance
is the perm. (Permeability is permeance times
thickness)
is calculated using Fick's law,
rearranged as Equation (1).
WVT = P · A · t · Δ(VP)
(1)
Where P is the system permeance
in units of weight/
(area · time · pressure), A
is area with the same area
units as used in permeance,
t is the time with the same
units used in permeance,
and Δ(VP) is the vapor pressure
difference in the same
units of pressure used in
permeance. WVT represents water
vapor transmission and will then be
in the same units of weight used in
permeance. In IP units, permeance is
usually expressed in units of grains of
moisture / (h·ft2· in. Hg) where a grain
is 1/7,000th of a pound.
For a low-permeance insulation
material with a permeability of p and
thickness T covered with a vapor retarder
jacket with a permeance P, the
insulation system permeance, Psys,
can be calculated using Equation (2).
Psys = 1 / (T / p + 1 / P)
(2)
For a given insulation material, selection
of insulation thickness is often
achieved using 3E Plus with the " Condensation
Control " Calculation Type,
for ambient conditions of the Ashrae
Design Dry-Bulb Temperature, a 90%
RH (relative humidity), and a 0-mi/h
wind speed (which is worst case). For
Tampa, Fla., for example, that design
temperature is given in the Ashrae
table as a 78.4°F dew point temperature.
Using either a psychrometric
chart or a calculator (and 3E Plus has
a built-in psychrometric calculator) for
90% RH, we can see that these conditions
are about equivalent to an 81°F
dry-bulb temperature. So, when using
3E Plus, the designer would input
81°F, 90% RH, and 0-mi/h wind speed
as the ambient conditions. For a given
pipe diameter, operating temperature,
and jacket type, the program will determine
the minimum thickness to
prevent surface condensation at these
conditions. Note that since the pipe is
located outdoors, the program is not
selecting an insulation thickness at
which surface condensation will never
occur. Rather, it is determining the
thickness to prevent surface condensa38
ChemiCal engineering www.Che.Com oCtober 2010
Figure 5. Shown here are two insulators installing
single-layer calcium silicate pipe insulation at
a petroleum refinery
tion with 0-mi/h wind and relative humidity
levels up to 90%. At higher relative
humidity levels and 0-mi/h wind,
surface condensations may well occur.
New MaterialS
aNd teChNologieS
Corrosion protection gel
To prevent CUI, there is a commercially
available compound, with a
thick, gel-like consistency, that can
be applied to the pipe surface prior to
being insulated (this gel material can
be used for process temperatures up
to 350°F). When properly installed, it
excludes both oxygen and water from
the surface of the pipe. In addition,
it has a chemically active ingredient
that reacts with steel to form a protective
layer. In tests, this material has
been shown to significantly reduce
external surface corrosion. Combined
with corrosion inhibitors contained in
some insulation materials, this solution
provides redundancy in preventing
CUI. For those with interest in
this material, see Ref. 2.
Aerogel insulation blankets
Aerogel blanket insulation has become
commercially available in the
last ten years. This type of insulation
has apparent thermal conductivities,
ka, in the range 0.10 to 0.15
Btu·in./h·ft2·°F at 75°F. Various forms
of aerogel insulation are designed for
applications from -300 to 1,200°F.
Characteristic ka versus temperature
data are available showing ka around
0.07 at -300°F to 0.70 at 1,200°F. Because
of the low ka, the insulation is
produced in thin sheets, 0.2- to 0.4-in.
thick and in rolls 5-ft wide and 100to
200-ft long. It is often installed by
spiral wrapping around pipes mul
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Chemical Engineering October 2010

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